A turbofan engine has at its core a turbojet drive engine, which, in addition to providing thrust, powers a large bypass fan that is located in front of the core engine. The bypass fan compresses air and passes it through a bypass duct which surrounds the core drive engine. The air compressed by the bypass fan exits the bypass duct to create additional thrust. For large thrust engines a turbofan is much more efficient than a turbojet engine. The bypass fan is driven by a turbine located at the exit of the core drive engine through a drive shaft which extends through substantially the full length of the core drive engine along its longitudinal center line. In addition, the core drive engine has a compressor at its inlet which is driven by another turbine located at the exit of the core drive engine through another drive shaft which extends through substantially the full length of the core drive engine along its longitudinal center line. Since these shafts are buried in the center of a turbojet engine, they become quite hot, which requires them to have expensive bearing systems and possibly even a cooling system of some type. This adds considerably to the cost of the engine.
In addition, a bypass fan has a large diameter which makes turning it with a starter motor difficult. Thus, the bypass fan often is disconnected from the core drive engine during start-up. This also adds to the cost and complexity of the engine. It also is necessary to place the starter motor at the front of the engine so that it is not subjected to the high temperature created by the core drive engine.
Finally, large conventional turbofan engines have large diameter fans. Due to the fact that fan tip speed must remain in the subsonic range the fan must rotate at a lower speed than the core drive engine. This requires gearing between the drive engine and the fan which further adds to the cost and complexity of the engine. In addition, it requires the use of high speed bearings for the fan. Moreover, the diameter of the fan is limited due to the problem of physically mounting the engine on an airplane. This, coupled with the fact that the larger the fan the larger the drive engine needs to be to power it, ultimately limits the bypass ratio that can be obtained.
The subject invention overcomes the foregoing shortcomings of the prior art turbofan engines by placing the bypass duct at the center of the engine and surrounding it with an annular drive engine.
In a preferred embodiment, this is accomplished by having an annular shell which carries the annular drive engine compressor and turbine. The outside of the annular drive engine is enclosed by an annular cover. A plurality of burner cans are located between the compressor and the turbine. The shell is attached to the tips of the blades of a bypass fan which is rotatably mounted on a central shaft. Thus, the shell, turbine, compressor and fan rotate together. Because the annular drive engine is not located at the center of the engine, the centrally mounted drive shafts are not heated by it and do not have to have complex bearings or cooling systems.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.